It's just so entertaining.
I love gaming in general, so if I play it myself or watch someone else play it, it doesn't matter.
And you get to know the people on a sort of personal level, and the way they interact with the game (or eachother if there's more than 1 person) can be entertaining or interesting + you get to experience the game at the same time.
And gaming is just so relaxing to sit and watch <3
I could sit and watch it all day. When MasaeAnela played through Tales of Vesperia, and I hadn't watched any of it yet, cause I didn't think I would like the game. But, one day, I had nothing to watch, so I sat down and started the let's play. It was just so good, so I watched around 90 episodes in 1-2 weeks time. Watching around 10 episodes a day. It was amazing.
It also made me buy a XBOX 360 just for that one game.